The present application relates generally to variable speed drives. The application relates more specifically to systems and methods for synchronous operation of variable speed drives having active converters.
A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications typically includes a rectifier or converter, a DC link, and an inverter. More recent advances in drive technology have introduced the concept of an active converter. The active converter provides the drive with the capability to provide sinusoidal input current loading to the power mains, alleviating issues with low order harmonic currents creating low order harmonic voltage distortion on the distribution voltage of the mains.
VSDs that incorporate active converter technology to provide power factor correction and reduced input current harmonics also generate a significantly higher level of common mode RMS and peak to peak voltage to the motor stator windings as compared to conventional VSDs. This common mode voltage can be coupled to the rotor of the motor via various stray machine capacitances, causing motor and compressor bearing fluting, and these common mode voltages which result in currents flowing through the machine bearings may cause premature bearing failures in the motor and/or compressor.
Proper operation of the active converter control methodology, using the synchronous d-q reference frame requires knowledge of the instantaneous phase angle of the input line-to-line voltage. If the reference frame angle is incorrect or unknown, then the input power factor and the harmonic distortion of the input current to the Variable Speed Drive (VSD) with Active Converter cannot be controlled properly. If the VSD is required to ride-through an extended loss of the input line-to-line voltage and re-synchronize to the input mains when the power is restored, a means to retain the expected d-q reference frame angle during the loss of mains is needed. In addition, a means to quickly lock back onto the input mains line-to-line voltage and generate the actual phase angle of the line-to-line voltage is required.
What are needed are a system and/or method that satisfy one or more of these needs or provides other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.